[TECHNICAL FIELD TO WHICH THE INVENTION BELONGS]
The present invention relates to a novel naphthoquinone derivative and to an electrophotographic material used in an image forming apparatus such as an electrostatic copying machine, a facsimile and a laser beam printer.
[PRIOR ART]
In image-forming apparatuses such as copying machines using the Carlson processing, a facsimile and a laser printer, electrophotographic materials using various materials have been used. The first of such electrophotographic materials is an inorganic photosensitive material in which an inorganic material such as selenium is used as a photosensitive layer, and the other is an organic photosensitive material using an organic material (OPC) as a photosensitive layer. The organic photosensitive material is cheaper than the inorganic photosensitive material and has higher productivity and is non-polluting. Hence, wide investigations have been carried out with regard to organic photosensitive materials.
Many organic photosensitive materials are so-called function-separating type photosensitive materials which are laminated-type photosensitive materials in which a charge generating layer and a charge transporting layer are laminated, but so-called single layer-type photosensitive materials obtained by dispersing a charge generating agent and a charge transporting agent in a single photosensitive layer are also known.
The charge transporting agent used in such a photosensitive material are required to have a high carrier movability. Since most of charge transporting agents having high carrier movability are positive hole transporting, the organic photosensitive materials are limited to negatively charged laminated-type organic photosensitive materials in which a charge transporting layer is provided at the outermost layer from the viewpoint of mechanical strength. However, because the negatively charged organic phososensitive material utilizes negatively polar corona dischage, the amount of ozone generated is large, and a problem of contaminating the environment or of deteriorating the photosensitive material occurs.
To remove such a problem, the use of an electron transporting agent as the charge transporting agent has been investigated, and for example, Japanese Laid-Open Patent Publication No. 206349/1989 proposes the use of a compound having a diphenoquinone structure or a benzoquinone structure as an electron transporting agent of an electrophotographic photosensitive material.
Japanese Laid-Open Patent Publication No. 110227/1994 proposes the use of a compound having a benzoquinone structure or a naphthoquinone structure as an electron trasporting agent for an electron photosensitive material.
However, conventional electron transporting agents such as generally diphenoquinone derivatives, benzoquinone derivatives, and naphthoquinone derivatives have been difficult to match with charge generating agents, and electron injection from charge generating agent to a charge transporting agent becomes insufficient. Such electron transporting agents have poor solubility in binder resins, the hopping distance becomes long, and movabillity of electrons in a low electric field is difficult to occur. Therefore, photosensitive materials containing conventional electron transporting agents, as is clear from the photosensitivity tests described in Examples, the residual potential becomes high, and the sensitivity is low.
As stated above, many of organic photosensitive materials now put to practical use contain a laminated-type photosensitive layer, but in comparison with the above type, photosensitive materials equipped with a single layer phosensitive layer has a simple structure and is easy to produce. In addition, it has many advantages, for example, it suppresses the occurrence of the film defects, and optical characteristics will be increased.
In addition, a photosensitive material equipped with a photosensitive layer of a single layer type conjointly uses an electron transporting agent and a positive hole transporting agent as a charge transporting agent can be used as a positive charging type and a negative charging type and the applicability range of the. photosensitive material can be extended. However, the conventional electron transporting agent and the positive hole transporting agent by their interaction poses a problem of obstructing the transporting of an electron and a positive hole. Hence, a photosensitive material equipped with a single layer type photosensitive layer has not been widely put into practical utility.
[SUMMARY OF THE INVENTION]
The object of this invention is to solve the above-mentioned technical problem, and to provide a preferred novel compound useful as an electron transporting agent in an electrophotographic material, and an electrophotographic material having the above novel compound which electrophotographic material has a higher sensitivity than heretofore.
The naphthoquinone derivative of this invention for solving the above problem is expressed by general formula (1) ##STR2## wherein R.sub.1 represents a hydrogen atom, an alkyl group which may have a substituent, or an aryl group which may have a substituent, R.sub.2 represents a hydrogen atom, an alkyl group which may have a substituent, a halogenated alkyl group, an aldehyde group, a nitro group, or a group (i): ##STR3##
The naphthoquinone derivative of this invention expressed by the general formula (1) has excellent electron acceptability by the action of a group which the naphthoquinone ring has, namely by the action of &gt;C.dbd.O, and by the action of the group R.sub.1, the naphthoquinone derivative of this invention has a good solubility in a solvent, and a good mutual solubility in a binder resin. Because the above compound of formula (1) is uniformly dispersed in a photosensitive material, the hopping distance of an electron is short, especially it has excellent electron transportability in a low electric field, and matching with a charge generating agent is excellent. Accordingly, by using the derivative (1) as an electron transporting agent in an electronphotographic material, a highly sensitive electrosensitive material can be provided.
Since the naphthoquinone derivative (1) of this invention is obtained by bonding two naphthoquinone rings which inherently have excellent electron transportability and electron acceptability by the action of the above substituents, it is surmised that an increase in electron transportability can be seen.
The naphthoquinone derivative (1) of this invention can also be used as a solar battery and an EL element by utilizing its high electron transportability.
The electrophotographic material of this invention is composed of a conductive substrate and a photosensitive layer provided thereon. This photosensitive layer is characterized in that it contains a naphthoquinone derivative represented by the general formula (1) of this invention.
The photosensitive layer containing the naphthoquinone layer (1) mentioned above has excellent transportability in a low electric field and the proportion of re-bonding an electron to a positive hole in the layer decreases and when the apparent occurrence of charging efficiency approaches an actual value, the sensitivity of the photosensitive material increases. Furthermore, the residual potential of the photosensitive material becomes low, and the stability and durabilility also increase at the time of repeating exposure.
Especially since the naphthoquinone derivative (1) does not cause the interaction with positive hole transporting agent, which hamper the transportation of an electron and a positive hole, when it is used in the photosensitive layer of the single layer-type containing the positive hole transporting agent in the same layer, it is possible to prepare a photosensitve body having high sensitivity.
When the photosensitive material contains an electron transporting agent having an oxidation-reduction potential of -0.8 to -1.4 V, the sensitivity of the photosensitive material is further increases.
Since the electron transporting agent extracts an electron from a charge generating agent and trasmits it to the naphthoquinone derivative (1), it is surmised that injection of an electron from the charge generating agent to the naphthoquinone derivative (1) will become smoother.
In view of affinity for the naphthoquinone derivative (1), the electron transporting agent may preferably be a diphenoquinone derivative of general formula (2) ##STR4## wherein R.sup.A, R.sup.B, R.sup.C and R.sup.D may be the same or different and each represents a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, an aralkyl group, a cycloalkyl group, or an amino group, or preferably a benzoquinone derivative of the general formula (3) ##STR5## wherein R.sup.E, R.sup.F, R.sup.G and R.sup.H may be the same or different, and each represents a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, an aralkyl group, a cyloalkyl group, or an amino group which may have a substituent.